1998 Force 40 Fuel Econ

[BUDXR7]

Just purchased a 1972 McKee craft, similar to Whaler, with a the Force 40 on it. The boat, engine, passengers and gear proabably checks in around 1250 lbs. Rough estimate is 5 miles per gallon @ varying speed and I have yet to bury the throttle. Once I plane her I keep it between 4000-4200.

I did nothing to the engine when I bought the boat and she runs like a top. I'll probably do plugs and a decarb on it soon. Possibly a carb rebuild over the winter.

Anybody else have any prop suggestions or econ numbers on thsi engine I can compare to?

 

[Firemedic09]

Re: 1998 Force 40 Fuel Econ

from what i understand force wasnt known for great fuel efficiency. keeping the throttle low will keep you GPH high. i have a force 125, this thirsty thing consumes anywhere around 5 to 6 GPH if its WOT it is around 8 GPH

 

[BUDXR7]

Re: 1998 Force 40 Fuel Econ

Okay, so I checked the prop size. There is absolutly no markings on it. I went over this thing with a magnifying glass. There is what appears to be a factory orginal sticker on the lower unit though with the gear ratio, 2:1, which I know to be correct and a prop size, 13X26. That seemed a bit extreme to me for a 40 hp motor. I will be using a GPS on it this weekend to get a speed measurement on it. It cant be more than 25.

 

[GrindKore]

Re: 1998 Force 40 Fuel Econ

Okay, so I checked the prop size. There is absolutly no markings on it. I went over this thing with a magnifying glass. There is what appears to be a factory orginal sticker on the lower unit though with the gear ratio, 2:1, which I know to be correct and a prop size, 13X26. That seemed a bit extreme to me for a 40 hp motor. I will be using a GPS on it this weekend to get a speed measurement on it. It cant be more than 25.

The 13x26 is too much of a prop for this motor, I have FORCE 90 and 13x19 is biggest prop it can swing comfotably on my 16' ski boat.

 

[roscoe]

Re: 1998 Force 40 Fuel Econ

13x26 is not a prop reference. it is the teeth on the gears in the lower unit.

26 ..... 13 .... 2 to 1

They don't even make a prop that size for that motor.

Sounds like your "fuel economy" is pretty good, normal.
Boats are not fuel efficient vehicles.

Fuel use is usually measured in gallons per hour.
Your 40 can use up to 4 gallons per hour at wide open throttle (WOT).
Should use about 2-2.5 gallons per hour at 3500 rpm.

Prop selection is determined by WOT rpm's.

What are your WOT rpm's ? If they are 5000, your prop is good.
If the rpm are over 5600, or under 4800, then you can consider a prop change.

If you don't have a tach, you are only guessing and wasting time and money.

The prop numbers are most likely on the inside edge or the mating end.

You need to go to the prop forum and read a little before switching props.

 

[BUDXR7]

Re: 1998 Force 40 Fuel Econ

Thanks Roscoe. 13X26, 2:1, sounds logical. I know it's a 13 tooth spline. Like I said though, there is absolutly zero markings on it. I did just install a tach myself. I don't know how much calibration these require beyond setting the number of poles though. There is a calibration screw. I've touched 5000 RPM and haven't buried the throttle yet. So the recommended 5250 WOT is probably what I can expect. Most likely the ideal prop for that boat.

I know engines are rated in GPH, but once I mate it to my boat(every one will be different), i'd like to know what the range is, so I'm going by GPM as well. Thanks for the rough estimates though, I can convert that to range.

 

[roscoe]

Re: 1998 Force 40 Fuel Econ

Gallons per hour will be pretty close for a given rpm, from boat to boat, if it is propped correctly for your boat.

Your range is going to be dependent on wind, river current/tide, and load.